1. Field of the Invention
The present invention relates to a semiconductor device such as a field-effect transistor (FET) using a heterojunction consisting of InP.
2. Related Background Art
InP has a saturated electron speed higher than that of GaAs or InGaAs and a thermal conductivity higher than that of GaAs. Therefore, an FET formed by using this InP is suitable as a high-power circuit element in an RF circuit. On the other hand, it is conventionally impossible to form a Schottky junction consisting of InP and having a good junction state. For this reason, a MIS heterojunction FET in which an insulating film such as an SiO.sub.2 film or an SiN film is formed on the junction surface has been developed.
In such a MISFET using InP, however, since the insulating layer is formed in the junction surface between a metal and a semiconductor, a high-concentration interface level is present in the interface between the semiconductor and the insulating layer. Therefore, due to this interface level, a drift occur in a drain current and a hysteresis in transfer characteristics occur in a low-frequency region.
In addition, a GaAs-based high electron mobility transistor (HEMT) using a two-dimensional electron gas formed in an AlGaAs/GaAs heterojunction interface is available, and an HEMT using a heterojunction consisting of InP has been studied.
This InP-based HEMT has, for example, a structure as shown in FIG. 1. This structure is disclosed in "Novel AlInAs/InP.HEMT" (ELECTRONICS LETTERS, May. 10, 1990, Vol. 26, No. 10, page 651). Referring to FIG. 1, an AlInAs layer 2 as a buffer layer and an InP layer 3 as an active layer are formed on an InP semiconductor substrate 1. An undoped AlInAs layer 4 is formed on the InP layer 3, and an n.sup.+ -type AlInAs layer 5 and an n.sup.+ -type InP layer 6 are formed on the AlInAs layer 4. A gate electrode 7 is formed on the AlInAs layer 5, and a drain electrode 8 and a source electrode 9 are formed on the InP layer 6. This HEMT using an AlInAs/InP heterojunction is also promising as a high-power circuit element for use in an RF circuit.
However, in such an HEMT using an n-AlInAs/InP heterojunction, compatibility between Al and P elements in the interface of AlInAs and InP is low, and crystal matching properties therebetween is poor. For this reason, in the InP-based HEMT, high-mobility electrons as those obtained in a GaAs-based HEMT cannot be obtained, and no good RF characteristics can be achieved.